Coupled styrene-isoprene-butadiene rubber

ABSTRACT

The subject invention discloses a process for preparing a coupled styrene-isoprene-butadiene rubber which is particularly valuable for use in making automobile tire tread rubber compounds which comprises the steps of (1) solution terpolymerizing in an organic solvent from about 5 weight percent to about 40 weight percent styrene, from about 1 weight percent to about 10 weight percent isoprene, and from about 50 weight percent to about 94 weight percent 1,3-butadiene, based on total monomers, to a conversion of at least about 90% to produce a living styrene-isoprene-butadiene polymer, wherein the terpolymerization is initiated with an organolithium compound, and wherein the terpolymerization is conducted in the presence of a polar modifier at a molar ratio of the polar modifier to the organolithium compound, which is within the range of about 0.5:1 to about 5:1, and wherein the terpolymerization is conducted at a temperature which is within the range of about 20° C. to about 150° C.; and (2) coupling the living styrene-isoprene-butadiene polymer with a coupling agent selected from the group consisting of tin tetrachloride and silicon tetrachloride, wherein the molar ratio of the coupling agent to the organolithium compound is within the range of about 3 to 1:6.

BACKGROUND OF THE INVENTION

Environmental concerns and fuel expenses are of major importance tomotorists in the world today and will probably be of growing concern aswe move toward the twenty-first century. In recent years, manymodifications have been implemented which make motor vehicles moreenergy efficient. For instance, better fuel efficiency is being attainedby implementing more aerodynamic designs which offer a lower coefficientof drag. Improved engine and transmission designs have also improved theoverall fuel efficiency of automobiles and trucks. Improved fuelefficiency can also be attained by designing tires which display lessrolling resistance. Accordingly, automobile owners are now demandingtires which exhibit low rolling resistance to attain the fuel economywhich they are seeking.

In order to reduce the rolling resistance of a tire, rubbers having ahigh rebound can be utilized in making the tires' treads. Tires madewith such rubbers undergo less energy loss during rolling. Thetraditional problem associated with this approach is that the tires' wettraction and wet skid resistance characteristics are compromised. Thisis because good rolling resistance which favors low energy loss and goodtraction characteristics which favor high energy loss areviscoelastically inconsistent properties. Good traction and wet skidresistance are, of course, extremely important characteristics for tiresto exhibit and compromising these attributes is generally unacceptable.

In order to balance these two viscoelastically inconsistent properties,mixtures of various types of synthetic and natural rubber are normallyutilized in tire treads. For instance, various mixtures ofstyrene-butadiene rubber and polybutadiene rubber are commonly used as arubbery material for automobile tire treads. However, such blends arenot totally satisfactory for all purposes. Numerous approaches have beentaken to balance these viscoelastically inconsistent properties withmixed results being attained.

U.S. Pat. No. 4,843,120 discloses that tires having improved performancecharacteristics can be prepared by utilizing rubbery polymers havingmultiple glass transition temperatures as the tread rubber. Theserubbery polymers having multiple glass transition temperatures exhibit afirst glass transition temperature which is within the range of about-110° C. to -20° C. and exhibit a second glass transition temperaturewhich is within the range of about -50° C. to 0° C. According to U.S.Pat. No. 4,843,120, these polymers are made by polymerizing at least oneconjugated diolefin monomer in a first reaction zone at a temperatureand under conditions sufficient to produce a first polymeric segmenthaving a glass transition temperature which is between -110° C. and -20°C. and subsequently continuing said polymerization in a second reactionzone at a temperature and under conditions sufficient to produce asecond polymeric segment having a glass transition temperature which isbetween -20° C. and 20° C.. Such polymerizations are normally catalyzedwith an organolithium catalyst and are normally carried out in an inertorganic solvent.

U.S. Pat. No. 5,137,998 discloses a process for preparing a rubberyterpolymer of styrene, isoprene and butadiene, having multiple glasstransition temperatures and having an excellent combination ofproperties for use in making tire treads which comprises:terpolymerizing styrene, isoprene and 1,3-butadiene in an organicsolvent at a temperature of no more than about 40° C. in the presence of(a) at least one member selected from the group consisting oftripiperidino phosphine oxide and alkali metal alkoxides and (b) anorganolithium compound.

U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outercircumferential tread where said tread is a sulfur cured rubbercomposition comprised of, based on 100 parts by weight rubber (phr), (A)about 10 to about 90 parts by weight of a styrene, isoprene, butadieneterpolymer rubber (SIBR), and (B) about 70 to about 30 weight percent ofat least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadienerubber wherein said SIBR rubber is comprised of (1) about 10 to about 35weight percent bound styrene, (2) about 30 to about 50 weight percentbound isoprene and (3) about 30 to about 40 weight percent boundbutadiene and is characterized by having a single glass transitiontemperature (Tg) which is in the range of about -10° C. to about -40° C.and, further the said bound butadiene structure contains about 30 toabout 40 percent 1,2-vinyl units, the said bound isoprene structurecontains about 10 to about 30 percent 3,4-units, and the sum of thepercent 1,2-vinyl units of the bound butadiene and the percent 3,4-unitsof the bound isoprene is in the range of about 40 to about 70 percent.

U.S. Pat. No. 5,272,220 and U.S. Pat. No. 5,317,062 disclose a processfor preparing a styrene-isoprene-butadiene rubber which is particularlyvaluable for use in making truck tire treads which comprises the stepsof (1) continuously solution terpolymerizing in an organic solvent fromabout 5 weight percent to about 20 weight percent styrene, from about 7weight percent to about 35 weight percent isoprene, and from about 55weight percent to about 88 weight percent 1,3-butadiene, based on totalmonomers, to a conversion which is in the range of about 60% to 100% toproduce a living intermediate polymer, wherein the terpolymerization isinitiated with an organolithium compound, wherein the terpolymerizationis conducted in the presence of 10 ppm to 500 ppm of 1,2-butadiene, andwherein the terpolymerization is conducted in the presence ofN,N,N',N'-tetramethylethylenediamine at a molar ratio ofN,N,N',N'-tetramethylethylenediamine to the organolithium compound whichis within the range of about 0.01:1 to about 0.2:1, and wherein theterpolymerization is conducted at a temperature which is within therange of about 75° C. to about 150° C.; (2) partially coupling theliving intermediate polymer with a coupling agent selected from thegroup consisting of divinyl benzene, tin tetrachloride and silicontetrachloride, wherein the molar ratio of the organolithium compound tothe coupling agent is within the range of about 6:1 to about 20:1; (3)allowing the terpolymerization to continue so as to produce thestyrene-isoprene-butadiene rubber; and recovering thestyrene-isoprene-butadiene rubber from the organic solvent.

U.S. Pat. No. 5,272,220 and U.S. Pat. No. 5,317,062 further disclose apneumatic truck tire having an outer circumferential tread wherein saidtread is a sulfur cured rubber composition comprised of, based on 100parts by weight of rubber, (a) from about 45 to about 75 parts of astyrene-isoprene-butadiene rubber comprised of repeat units which arederived from about 5 weight percent to about 20 weight percent styrene,from about 7 weight percent to about 35 weight percent isoprene, andfrom about 55 weight percent to about 88 weight percent 1,3-butadiene,wherein the repeat units derived from styrene, isoprene, and1,3-butadiene are in essentially random order, wherein from about 25% toabout 40% of the repeat units derived from the 1,3-butadiene are of thecis-microstructure, wherein from about 40% to about 60% of the repeatunits derived from the 1,3-butadiene are of the transmicrostructure,wherein from about 5% to about 25% of the repeat units derived from the1,3-butadiene are of the vinyl-microstructure, wherein from about 75% toabout 90% of the repeat units derived from the isoprene are of the1,4-microstructure, wherein from about 10% to about 25% of the repeatunits derived from the isoprene are of the 3,4-microstructure, whereinthe rubber has a glass transition temperature which is within the rangeof about -90° C. to about -70° C., wherein the rubber has a numberaverage molecular weight which is within the range of 150,000 to400,000, wherein the rubber has a weight average molecular weight of300,000 to 800,000, and wherein the rubber has an inhomogeneity which iswithin the range of 0.5 to 1.5; and (b) from about 25 to about 55 partsof natural rubber.

Coupled polymers are frequently utilized in tire tread rubber compounds.This is because coupled polymers provide improved processability, lowerhysteresis and improved filler-polymer interactions as compared to theiruncoupled counterparts. It is well known that the efficiency of couplingagents decreases substantially if the polymer live ends are styryllithium anions.

Styryl lithium anions are normally at the living chain ends ofstyrene-butadiene rubbers which are synthesized by anionicpolymerizations in the presence of polar modifiers. To overcome thisproblem, it is known that a small amount of additional 1,3-butadienemonomer can be added to cap the living styryl anion prior to coupling.However, the addition of more 1,3-butadiene monomer after the initialpolymerization has been completed but prior to coupling has certaindisadvantages. For instance, the introduction of additional1,3-butadiene monomer into the polymerization medium can increase thelevel of impurities present in the system. The introduction ofadditional 1,3-butadiene monomer also represents an additionalprocessing step which, of course, on a commercial basis adds to theultimate cost of the polymer.

SUMMARY OF THE INVENTION

By utilizing the techniques of this invention, tire tread rubberformulations can be prepared which exhibit outstanding wet traction, wetskid resistance, treadwear and rolling resistance characteristics.Because the styrene-isoprene-butadiene rubbers of this invention exhibitexcellent interaction with carbon black and silicon fillers, they arealso capable of being processed very easily. In other words, they areeasily processable into tire tread rubber compounds.

This invention is based upon the unexpected finding thatstyrene-isoprene-butadiene rubbers which are terminated with isoprenerepeat units can be synthesized employing lithium initiators and polarmodifiers without the necessity of a separate isoprene monomer additionstep. By employing the techniques of this invention, essentially all ofthe living chain ends in the polymer are terminated with isoprene repeatunits. This is true even in the case of batch techniques where all ofthe monomers are charged into the polymerization medium beforeinitiating the polymerization. Because virtually all of the living chainends in the polymer are terminated with isoprene, it can be readilycoupled with tin tetrachloride or silicon tetrachloride coupling agents.

After being coupled, the styrene-isoprenebutadiene terpolymer (SIBR) hasa high level of affinity for carbon black and silicon fillers. Tintetrachloride is the preferred coupling agent in cases where the use ofsilicon fillers is contemplated and silicon tetrachloride is preferredin cases where it is anticipated that carbon black will be employed asthe filler. This is because better interaction between the SIBR of thisinvention and silicon fillers is realized in cases where the SIBR iscoupled with tin tetrachloride. On the other hand, better interactionbetween the SIBR of this invention and carbon black fillers is realizedin cases where the SIBR is coupled with silicon tetrachloride.

It has been unexpectedly found that the rolling resistance and treadwear characteristics of automobile tires can be significantly improvedby incorporating the styrene-isoprene-butadiene rubber (SIBR) of thisinvention into the treads thereof. More importantly, this improvement inrolling resistance and tread wear characteristics can be achievedwithout sacrificing wet traction and wet skid resistance.

The SIBR of this invention is prepared by solution polymerizationsutilizing an organolithium initiator. The process used in synthesizingthis SIBR is conducted as a batch or continuous process which is carriedout at a temperature which is within the range of about 20° C. to about150° C. In cases where the polymerization is conducted employing acontinuous process, it will be necessary to utilize a multiple reactorsystem to ensure that the polymerization has been carried out to aconversion where isoprene repeat units are at the end of virtually allof the polymer chains. It has been found that gel build-up can beinhibited by conducting such polymerizations in the presence of1,2-butadiene and N,N,N',N'-tetramethylethylenediamine.

This invention more specifically reveals a process for preparing acoupled styrene-isoprene-butadiene rubber which is particularly valuablefor use in making automobile tire tread rubber compounds which comprisesthe steps of (1) solution terpolymerizing in an organic solvent fromabout 5 weight percent to about 40 weight percent styrene, from about 1weight percent to about 10 weight percent isoprene, and from about 50weight percent to about 94 weight percent 1,3-butadiene, based on totalmonomers, to a conversion of at least about 90% to produce a livingstyrene-isoprene-butadiene polymer, wherein the terpolymerization isinitiated with an organolithium compound, and wherein theterpolymerization is conducted in the presence of a polar modifier at amolar ratio of the polar modifier to the organolithium compound which iswithin the range of about 0.5:1 to about 5:1, and wherein theterpolymerization is conducted at a temperature which is within therange of about 20° C. to about 150° C.; and (2) coupling the livingstyrene-isoprene-butadiene polymer with a coupling agent selected fromthe group consisting of tin tetrachloride and silicon tetrachloride,wherein the molar ratio of the coupling agent to the organolithiumcompound is within the range of about 1:3 to 1:6.

The subject invention further discloses a styrene-isoprene-butadienerubber which is particularly valuable for use in making tire treads,said rubber being comprised of repeat units which are derived from about5 weight percent to about 40 weight percent styrene, from about 1 weightpercent to about 10 weight percent isoprene, and from about 50 weightpercent to about 94 weight percent 1,3-butadiene, wherein the repeatunits derived from styrene, isoprene, and 1,3-butadiene are inessentially random order, wherein the rubber has a glass transitiontemperature which is within the range of about -20° C. to about -45° C.,and wherein over 90% of the repeat units in the SIBR which are derivedfrom styrene are in blocks of containing less than three repeat units,wherein the rubber is coupled with a member selected from the groupconsisting of tin tetrachloride and silicon tetrachloride, and whereinthe rubber has a number average molecular weight which is within therange of about 250,000 to about 400,000.

This invention also reveals a sulfur curable rubber composition which isparticularly useful for making tire treads, said rubber compositionbeing comprised of (a) a styrene-isoprene-butadiene rubber which isparticularly valuable for use in making tire treads, said rubber beingcomprised of repeat units which are derived from about 5 weight percentto about 40 weight percent styrene, from about 1 weight percent to about10 weight percent isoprene, and from about 50 weight percent to about 94weight percent 1,3-butadiene, wherein the repeat units derived fromstyrene, isoprene, and 1,3-butadiene are in essentially random order,wherein the rubber has a glass transition temperature which is withinthe range of about -20° C. to about -45° C., and wherein over 90% of therepeat units in the SIBR which are derived from styrene are in blocks ofcontaining less than three repeat units, wherein the rubber is coupledwith a member selected from the group consisting of tin tetrachlorideand silicon tetrachloride, and wherein the rubber has a number averagemolecular weight which is within the range of about 250,000 to about400,000; (b) sulfur, (c) carbon black, (d) a silica filler, (e) anaccelerator, and (f) at least one additional sulfur curable rubber.

DETAILED DESCRIPTION OF THE INVENTION

The SIBR of this invention is synthesized by solution polymerization.Such solution polymerizations will normally be carried out in ahydrocarbon solvent which can be one or more aromatic, paraffinic orcycloparaffinic compounds. These solvents will normally contain from 4to 10 carbon atoms per molecule and will be liquids under the conditionsof the polymerization. Some representative examples of suitable organicsolvents include pentane, isooctane, cyclohexane, normal hexane,benzene, toluene, xylene, ethylbenzene, and the like, alone or inadmixture.

In the solution polymerizations of this invention, there will normallybe from about 5 to about 35 weight percent monomers in thepolymerization medium. Such polymerization media are, of course,comprised of the organic solvent, 1,3-butadiene monomer, styrene monomerand isoprene monomer. In most cases, it will be preferred for thepolymerization medium to contain from 10 to 30 weight percent monomers.It is generally more preferred for the polymerization medium to contain20 to 25 weight percent monomer.

The monomer charge compositions utilized in the polymerizations of thisinvention will typically contain from about 5 weight percent to about 40weight percent styrene, from about 1 weight percent to about 10 weightpercent isoprene and from about 50 weight percent to about 94 weightpercent 1,3-butadiene monomer. It is typically preferred for the monomercharge composition to contain from about 20 weight percent to about 35weight percent styrene, from about 2 weight percent to about 6 weightpercent isoprene, and from about 59 weight percent to about 78 weightpercent 1,3-butadiene. It is generally more preferred for the monomercharge composition to include from about 25 weight percent to about 30weight percent styrene, from about 3 weight percent to about 5 weightpercent isoprene, and from about 65 weight percent to about 72 weightpercent 1,3-butadiene.

The SIBR of this invention can be synthesized on a batch or continuousbasis. In batch processes, all of the monomers are generally chargedinto a single reactor with the polymerization therein being started bythe addition of an organolithium initiator. In such batch processes, thepolymerization is allowed to continue in the reactor until a highconversion of at least about 98 percent is attained. It is preferred forthe monomer conversion to be at least 99 percent and more preferred forthe monomer conversion to be in excess of 99.5 percent.

In continuous processes, the monomers and an organolithium initiator arecontinuously fed into the first reactor of a multiple reaction vesselsystem. In such multiple reaction vessel systems, it is important forthe monomer conversion in the last reactor to attain a high conversionof at least about 98 percent. It is preferred for the monomer conversionto be at least 99 percent and more preferred for the monomer conversionto be in excess of 99.5 percent.

In both batch and continuous polymerizations, the pressure in thereaction vessel is typically sufficient to maintain a substantiallyliquid phase under the conditions of the polymerization reaction. Thereaction medium will generally be maintained at a temperature which iswithin the range of about 20° C. to about 150° C. throughout theterpolymerization. This is generally preferred for the terpolymerizationto be conducted at a temperature which is within the range of about 60°C. to about 120° C. It is typically more preferred for theterpolymerization to be conducted at a temperature which is within therange of about 80° C. to about 100° C.

The organolithium compounds which can be utilized as initiators in theterpolymerizations of this invention include organomonolithium compoundsand organomonofunctional lithium compounds. The organo multifunctionallithium compounds will typically be organodilithium compounds ororganotrilithium compounds. Some representative examples of suitablemultifunctional organolithium compounds include 1,4-dilithiobutane,1,10-dilithiodecane, 1,20-dilithioeicosane, 1,4-dilithiobenzene,1,4-dilithionaphthalene, 9,10-dilithioanthracene,1,2-dilithio-1,2-diphenylethane, 1,3,5-trilithiopentane,1,5,15-trilithioeicosane, 1,3,5-trilithiocyclohexane,1,3,5,8-tetralithiodecane, 1,5,10,20-tetralithioeicosane,1,2,4,6-tetralithiocyclohexane, 4,4'-dilithiobiphenyl, and the like.

The organolithium compounds which can be utilized are normallyorganomonolithium compounds. The organolithium compounds which arepreferred can be represented by the formula: R--Li, wherein R representsa hydrocarbyl radical containing from 1 to about 20 carbon atoms.Generally, such monofunctional organolithium compounds will contain from1 to about 0 carbon atoms. Some representative examples of organolithiumcompounds which can be employed include methyllithium, ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, n-octyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-napthyllithium,4-butylphenyllithium, p-tolyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, and4-cyclohexylbutyllithium.

The amount of organolithium initiator employed will be dependent uponthe molecular weight which is desired for the SIBR being synthesized. Anamount of organolithium initiator will be selected to result in theproduction of SIBR having a number average molecular weight which iswithin the range of about 50,000 to about 200,000. The amount oforganolithium initiator will preferably be selected to result in theproduction of a SIBR having a number average molecular weight which iswithin the range of about 75,000 to 150,000. The amount of organolithiuminitiator will more preferably be selected to result in the productionof a SIBR having a number average molecular weight which is within therange of about 85,000 to 120,000. The number average molecular weightsreported in this paragraph are before coupling.

As a general rule in all anionic polymerizations, the molecular weightof the polymer produced is inversely proportional to the amount ofcatalyst utilized. As a general rule, from about 0.01 to about 1 phm(parts per hundred parts of monomer by weight) of the organolithiumcompound will be employed. In most cases, it will be preferred toutilize from about 0.015 to about 0.1 phm of the organolithium compoundwith it being most preferred to utilize from about 0.025 phm to 0.07 phmof the organolithium compound.

To inhibit gelation, it is important to carry out such polymerizationsin the presence of 1,2-butadiene andN,N,N',N'-tetramethylethylenediamine (TMEDA). For this reason,1,2-butadiene and TMEDA will also be continuously fed into the reactionvessel utilized. The 1,2-butadiene will typically be present in thepolymerization medium at a concentration which is within the range of 10to about 500 ppm (parts per million parts). It is generally preferredfor the 1,2-butadiene to be present at a level which is within the rangeof about 50 ppm to about 300 ppm. It is generally more preferred for the1,2-butadiene to be present at a level which is within the range ofabout 100 ppm to about 200 ppm.

The TMEDA acts as a polymerization modifier and increases the glasstransition temperature of the SIBR produced. It will accordingly beemployed, either alone or in conjunction with other polar modifiers, inan amount which will result in the SIBR being synthesized having thedesired high glass transition temperature. However, to be effective as agel inhibitor, the TMEDA will typically be present at a molar ratio ofTMEDA to the organolithium compound of at least about 0.01:1. It willnormally be preferred for a molar ratio of TMEDA to the organolithiumcompound of at least about 0.1:1 to be employed. It will normally bemore preferred for a molar ratio of TMEDA to the organolithium compoundof at least about 0.2:1 to be employed.

The glass transition temperature of the SIBR being synthesized will becontrolled to be within the range of about -18° C. to about -48° C. Thisgenerally requires a molar ratio of the polar modifier to theorganolithium which is within the range of about 0.5:1 to about 5:1.More typically, the molar ratio of the polar modifier to theorganolithium will be within the range of about 1:1 to about 4:1 toattain the desired glass transition temperature. A very high glasstransition temperature is preferred because it provides tire treadcompounds with good traction characteristics. However, if polymershaving glass transition temperatures higher than about -20° C. aresynthesized, the level of polar modifier required will be so high thatit will unduly interfere with coupling. In any case, the SIBR terpolymerwill typically have a glass transition temperature which is within therange of about -25° C. to about -40° C. and will more preferably have aglass transition temperature which is within the range of about -30° C.to about -35° C..

After the desired high monomer conversion of at least about 98% isachieved, the living polymer is coupled with tin tetrachloride orsilicon tetrachloride. This is typically done in a second reactionvessel. For instance, in the case of a continuous polymerization, theliving intermediate polymer can be pumped from the final polymerizationreaction vessel to a vessel where the coupling agent is added to thepolymerization medium. The coupling agent is added after a monomerconversion of at least about 98% has been attained and is morepreferably added after a monomer conversion of 99% has been attained.

The coupling agent is added at a level which is sufficient to jump themolecular weight of the polymer to the desired degree. A high level ofcoupling of the SIBR terpolymer chains is very desirable. As a generalrule, the molar ratio of coupling agent to the organolithium compoundwill be within the range of about 1:3 to about 1:6. Molar ratios of thecoupling agent to the organolithium compound which are within the rangeof about 2:7 to about 1:5 are preferred because they induce the highestlevel of coupling to achieve the desired increased in molecular weight.

The coupling increases the Mooney viscosity of the SIBR to be within therange of about 70 to about 120. This high Mooney viscosity providestires which are manufactured utilizing tire tread compounds containingthe SIBR to have outstanding rolling resistance and treadwearcharacteristics. To achieve these highly desirable objectives, it isbeneficial for the Mooney viscosity of the SIBR to be as high aspossible. However, the possibility of the rubber becomes more difficultas its Mooney viscosity increases. Accordingly, the Mooney viscosity ofthe rubber will typically not be in excess of about 120. On the otherhand, to get the low rolling resistance and outstanding tread wear, itis important for the Mooney viscosity of the SIBR to be at least 70 andpreferably higher. The SIBR of this invention will more typically have aMooney viscosity which is within the range of about 80 to about 115 andwill preferably have a Mooney viscosity which is within the range of 90to 110.

After being coupled, the SIBR produced is recovered from the organicsolvent. The SIBR can be recovered from the organic solvent by standardtechniques, such as decantation, filtration, centrification and thelike. It is often desirable to precipitate the SIBR from the organicsolvent by the addition of lower alcohols containing from 1 to about 4carbon atoms to the polymer solution. Suitable lower alcohols forprecipitation of the SIBR from the polymer cement include methanol,ethanol, isopropyl alcohol, n-propyl alcohol and t-butyl alcohol. Theutilization of lower alcohols to precipitate the SIBR from the polymercement also "kills" the living SIBR chains by inactivating lithium endgroups. After the SIBR is recovered from the organic solvent, steamstripping can be employed to reduce the level of volatile organiccompounds in the rubber.

The SIBR made by the process of this invention is characterized by beingcomprised of repeat units which are derived from about 5 weight percentto about 40 weight percent styrene, from about 1 weight percent to about10 weight percent isoprene, and from about 50 weight percent to about 94weight percent 1,3-butadiene, wherein the repeat units derived fromstyrene, isoprene, and 1,3-butadiene are in essentially random order,wherein the rubber has a glass transition temperature which is withinthe range of about -20° C. to about -45° C., and wherein over 90% of therepeat units in the SIBR which are derived from styrene are in blocks ofcontaining less than three repeat units. The repeat units in the SIBRwill preferably be derived from about 20 weight percent to about 35weight percent styrene, from about 2 weight percent to about 6 weightpercent isoprene, and from about 59 weight percent to about 78 weightpercent 1,3-butadiene. The repeat units in the SIBR will most preferablybe comprised of repeat units which are derived from about 25 weightpercent to about 30 weight percent styrene, from about 3 weight percentto about 5 weight percent isoprene, and from about 65 weight percent toabout 72 weight percent 1,3-butadiene. These repeat units which arederived from isoprene, styrene or 1,3-butadiene differ from the monomerfrom which they were derived in that a double bond was consumed by thepolymerization reaction.

The repeat units derived from styrene, isoprene and 1,3-butadiene are inthe SIBR in an essentially random order. However, the SIBR is not trulyrandom in that the polymer chains are virtually always terminated withisoprene repeat units. The term "random" as used herein means that therepeat units which are derived from styrene are well dispersedthroughout the polymer and are mixed in with repeat units which arederived from isoprene and 1,3-butadiene. It has been determined thatover 70% of the styrene in the SIBR is present in blocks of only onestyrene repeat unit. Over 90% of the repeat units in the SIBR which arederived from styrene are in blocks of one or two repeat units. Over 95%of the styrene in the SIBR is present in blocks of three or less repeatunits. Over 97% of the styrene present in the SIBR is present in blocksof four or less repeat units. Over 99% of the styrene present in theSIBR is present in blocks of five or less repeat units. Virtually 100%of the styrene present in the SIBR is in blocks of six or less repeatunits. As has been previously indicated, virtually all of the SIBRpolymer chains are terminated with isoprene repeat units, by this it ismeant that over 98% of the SIBR chains are terminated with isoprenerepeat units. Preferably, over 99% of the SIBR polymer chains areterminated with isoprene repeat units.

Preferably, from about 10% to about 20% of the repeat units derived fromthe 1,3-butadiene are of the cis-microstructure. Preferably, from about15% to about 30% of the repeat units derived from the 1,3-butadiene areof the trans-microstructure. Preferably, from about 55% to about 70% ofthe repeat units derived from the 1,3-butadiene are of thevinyl-microstructure. Preferably, from about 15% to about 30% of therepeat units derived from the isoprene are of the 1,4-microstructure.Preferably, from about 70% to about 85% of the repeat units derived fromthe isoprene are of the 3,4-microstructure.

After coupling, the SIBR will preferably have a number average molecularweight which is within the range of about 250,000 to about 400,000. Itis preferred for the SIBR to have a weight average molecular weightwhich is within the range of about 500,000 to about 600,000. It ispreferred for the SIBR to have an inhomogeneity (u) which is within therange of about 0.8 to 1.2. Inhomogeneity is defined by the equation:##EQU1## In other words, the ratio of the weight average molecularweight of the SIBR to its number average molecular weight is preferably2:1.

For purposes of this patent application, polymer microstructures aredetermined by nuclear magnetic resonance spectrometry (NMR). Glasstransition temperatures are determined by differential scanningcalorimetry at a heating rate of 10° C. per minute and molecular weightsare determined by gel permeation chromatography (GPC).

There are valuable benefits associated with utilizing the SIBRterpolymers of this invention in making tire tread compounds. Such tiretread compounds are blends of the coupled SIBR with one or moreadditional sulfur curable elastomers. For instance, the SIBR can beblended with natural rubbers and, optionally, high cis 1,4-polybutadienein making tire tread confounds. One such tread compound is comprised of,based on 100 parts by weight of rubber, (a) from about 45 parts to about75 parts of the SIBR and (b) from about 25 parts to about 55 parts ofnatural rubber. It is preferred for this tread compound to contain fromabout 55 parts to about 65 parts of the SIBR and from about 35 parts toabout 45 parts of natural rubber.

Another highly preferred blend for utilization in making automobiletires is comprised of, based on 100 parts by weight of rubber, (a) 50parts to 70 parts of the SIBR, (b) from about 15 parts to about 45 partsof polyisoprene rubber and (c) from about 2 parts to about 20 parts ofhigh cis 1,4-polybutadiene. It is preferred for this rubber blend tocontain from about 55 parts to about 65 parts of the SIBR, from about 5parts to about 15 parts of the high cis 1,4-polybutadiene, and fromabout 25 parts to about 40 parts of polyisoprene rubber.

Another highly preferred blend for utilization in making automobiletires is comprised of, based on 100 parts by weight of rubber, (a) 50parts to 70 parts of the SIBR and (b) from about 30 parts to about 50parts of high cis 1,4-polybutadiene. It is preferred for this rubberblend to contain from about 55 parts to about 65 parts of the SIBR andfrom about 5 parts to about 15 parts of the high cis 1,4-polybutadienerubber.

The high cis 1,4-polybutadiene utilized in such blends typically has amicrostructure wherein at least 80% of the butadiene repeat units arecis 1,4-isomeric units. In most cases, the high cis 1,4-polybutadienewill contain at least about 90% cis 1,4-isomeric butadiene units. Thehigh cis 1,4-polybutadiene can be prepared by solution polymerizationutilizing a catalyst consisting of (1) an organoaluminum compound, (2)an organonickel compound and (3) a hydrogen fluoride complex asdescribed in U.S. Pat. No. 3,856,764.

For instance, the SIBR rubber can be blended with natural rubber to maketread compounds for passenger tires which exhibit outstanding rollingresistance, traction and tread wear characteristics. Such blends willnormally contain from about 5 to about 40 weight percent natural rubberand from about 60 to about 95% of the segmented elastomer. Highperformance tires which exhibit very exceptional tractioncharacteristics, but somewhat comprised tread wear, can be prepared byblending the SIBR elastomer with solution or emulsion styrene-butadienerubber (SBR). In cases where tread wear is of greater importance thantraction, high cis-1,4-polybutadiene can be substituted for the SBR. Inany case, the segmented rubbers of this invention can be used to improvethe traction, tread wear and rolling resistance of tires made therewith.

These SIBR containing blends can be compounded utilizing conventionalingredients and standard techniques. For instance, the SIBR containingblends will typically be blended with carbon black and/or silicafillers, sulfur, accelerators, oils, waxes, scorch inhibiting agents andprocessing aids. In most cases, the SIBR containing rubber blends willbe compounded with sulfur and/or a sulfur containing compound, at leastone filler, at least one accelerator, at least one antidegradant, atleast one processing oil, zinc oxide, optionally a tackifier resin,optionally a reinforcing resin, optionally one or more fatty acids,optionally a peptizer and optionally one or more scorch inhibitingagents. Such blends will normally contain from about 0.5 to 5 phr (partsper hundred parts of rubber by weight) of sulfur and/or a sulfurcontaining compound with 1 phr to 2.5 phr being preferred. It may bedesirable to utilize insoluble sulfur in cases where bloom is a problem.

Normally from 10 to 150 phr of at least one filler will be utilized inthe blend with 30 to 80 phr being preferred. In most cases, at leastsome carbon black will be utilized in the filler. The filler can, ofcourse, be comprised totally of carbon black. Silica can be included inthe filler to improve tear resistance and heat build-up. Clays and/ortalc can be included in the filler to reduce cost.

The commonly employed siliceous pigments used in rubber compoundingapplications can be used as the silica in this invention, includingpyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate,e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,page 304 (1930) .

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300.

The silica might be expected to have an average ultimate particle size,for example, in the range of 0.01 to 0.05 micron as determined by theelectron microscope, although the silica particle may be even smaller,or possibly larger, in size.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc; silicas available fromRhone-Poulenc, with, for example, designations of Z1165MP and Z165GR andsilicas available from Degussa AG with, for example, designations VN2and VN3, etc. The PPG Hi-Sil silicas are currently preferred.

The blend will also normally include from 0.1 to 2.5 phr of at least oneaccelerator with 0.2 to 1.5 phr being preferred. Antidegradants, such asantioxidants and antiozonants, will generally be included in the blendin amounts ranging from 0.25 to 10 phr with amounts in the range of 1 to5 phr being preferred. Processing oils will generally be included in theblend in amounts ranging from 2 to 100 phr with amounts ranging from 5to 50 phr being preferred. The SIBR containing blends of this inventionwill also normally contain from 0.5 to 10 phr of zinc oxide with 1 to 5phr being preferred. These blends can optionally contain from 0 to 10phr of tackifier resins, 0 to 10 phr of reinforcing resins, 1 to 10 phrof fatty acids, 0 to 2.5 phr of peptizers, and 0 to 1 phr of scorchinhibiting agents.

The SIBR containing rubber blends of this invention can be used in tiretreads in conjunction with ordinary tire manufacturing techniques. Tiresare built utilizing standard procedures with the SIBR simply beingsubstituted for the rubber compounds typically used as the tread rubber.After the tire has been built with the SIBR containing blend, it can bevulcanized using a normal tire cure cycle. Tires made in accordance withthis invention can be cured over a wide temperature range. However, itis generally preferred for the tires of this invention to be cured at atemperature ranging from about 132° C. (270° F.) to about 166° C. (330°F.). It is more typical for the tires of this invention to be cured at atemperature ranging from about 143° C. (290° F.) to about 154° C. (310°F.). It is generally preferred for the cure cycle used to vulcanize thetires of this invention to have a duration of about 10 to about 14minutes with a cure cycle of about 12 minutes being most preferred.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, all parts and percentages aregiven by weight.

Example 1

In this experiment, 2,300 grams of a silica/alumina/molecular sieve/NaOHdried premix containing 19.2 weight percent styrene/isoprene/butadienein hexane was charged into a one gallon (3.8 liters) reactor. The ratioof styrene to isoprene and to 1,3-butadiene was 30/5/65. After thescavenger level of 2 ppm was determined, 0.79 ml of neat TMEDA(N,N,N',N'-tetramethylethylenediamine; 6.63 M) and 3.59 ml of a 1.04 Msolution n-butyl lithium (in hexane; 3.47 ml for initiation and 0.24 mlfor scavenging the premix) was added to the reactor. The molar ratio ofmodifier to n-butyl lithium was 1.5. The polymerization was allowed toproceed at 70° C. for 1.5 hours. Analysis of the residual monomerscontained in the polymerization mixture by gas: chromatograph indicatedthat the polymerization was 99.8% complete at this time. Thepolymerization was continued for another 30 minutes to assure 100%conversion. A small portion of the reaction mixture (300 grams; the basepolymer) was then removed from the reactor and stabilized with anethanol/antioxidant mixture. The remainder of the reaction mixture wasthen coupled using a coupling agent, SnCl4 (tin tetrachloride), at a0.25/1 ratio to n-butyl lithium. Thus, 0.75 ml of a 1M solution of SnCl4solution (in hexane) was added to the reactor and the coupling reactionwas proceeded at 70° C. for 30 minutes. Then, 1 ml of ethanol was addedto the reactor to shortstop the polymerization and polymer was removedfrom the reactor and stabilized with 1 phm of antioxidant. Afterevaporating hexane, both base and coupled polymers were dried in avacuum oven at 50° C. The coupled 30/5/65 SIBR was determined to have aglass transition temperature (Tg) at -22° C. and have a microstructurewhich contained 38% 1,2-polybutadiene units, 27% 1,4-polybutadieneunits, 3% 3,4polyisoprene units, 2% 1,4-polyisoprene units and 30%random polystyrene units. The Mooney viscosities of the base and coupled30/5/65 SIBRs were 18 and 72, respectively.

Comparative Example 2

The procedure described in Example 1 was utilized in this example exceptthat a 30/70 mixture of styrene/1,3-butadiene was used as the premix.The coupled 30/70 SBR was determined to have a Tg at -22° C. and have amicrostructure which contained 41% 1,2polybutadiene units, 29%1,4-polybutadiene units and 30% random polystyrene units. The Mooneyviscosities of the base and coupled 30/70 SBRs were 25 and 62,respectively.

While certain representative embodiments and details have been shown forthe purpose of illustrating the subject invention, it will be apparentto those skilled in this art that various changes and modifications canbe made therein without departing from the scope of the subjectinvention.

What is claimed is:
 1. A process for preparing a coupledstyrene-isoprene-butadiene rubber which is particularly valuable for usein making automobile tire tread rubber compounds which comprises thesteps of (1) solution terpolymerizing in an organic solvent from about 5weight percent to about 40 weight percent styrene, from about 1 weightpercent to about 10 weight percent isoprene, and from about 50 weightpercent to about 94 weight percent 1,3-butadiene, based on totalmonomers, to a conversion of at least about 90% to produce a livingstyrene-isoprene-butadiene polymer, wherein the terpolymerization isinitiated with an organolithium compound, and wherein theterpolymerization is conducted in the presence of a polar modifier at amolar ratio of the polar modifier to the organolithium compound which iswithin the range of about 0.5:1 to about 5:1, and wherein theterpolymerization is conducted at a temperature which is within therange of about 20° C. to about 150° C.; and (2) coupling the livingstyrene-isoprene-butadiene polymer with a coupling agent selected fromthe group consisting of tin tetrachloride and silicon tetrachloride toattain Mooney viscosity which is within the range of about 70 to about120, wherein the molar ratio of the coupling agent to the organolithiumcompound is within the range of about 1:3 to 1:6.
 2. A process asspecified in claim 1 wherein the polar modifier isN,N,N',N'-tetramethylethylenediamine.
 3. A process as specified in claim2 wherein the terpolymerization is conducted in the presence of1,2-butadiene at a level which is within the range of about 50 ppm toabout 300 ppm.
 4. A process as specified in claim 3 wherein the molarratio of N,N,N',N'-tetramethylethylenediamine to organolithium initiatoris within the range of about 1:1 to about 4:1.
 5. A process as specifiedin claim 4 wherein the molar ratio of the coupling agent to theorganolithium initiator is within the range of about 2:7 to about 1:5.6. A process as specified in claim 5 wherein the 1,2-butadiene ispresent at a level which is within the range of about 100 ppm to about200 ppm.
 7. A process as specified in claim 6 wherein the organolithiumcompound is an organomonolithium compound.
 8. A process as specified inclaim 7 wherein the living polymer is partially coupled after a monomerconversion of at least about 98% has been attained.
 9. A process asspecified in claim 8 wherein the coupling agent is tin tetrachloride.